Weathervaning - what actually turns an aircraft in flight.

[alfadog]

From another thread, the suggestion was made to start a new thread. I would like to get some feedback on my opposition to what most folks, if not all, are taught is the cause of an airplane turning in flight.

I have often taken exception with the concept that banking the airplane causes it to turn. That would caused only translational movement with the nose still pointed straight ahead if not for something else at work. Linear forces in different directions acting on the same point (the center of lift) cause a resultant translation, not rotation. Rotation requires a force couple, two opposite forces separated by a moment arm. What turns an airplane, IMO, is weathervaning, i.e. the force couple caused by the horizontal component of lift in a bank opposed by the center of sidewise resistance to the air being well behind the center of lift, this caused by the abundance of side surface area to the rear. Rich Stowell likes to say that the elevator turns the airplane but it would be more correct to say the empennage and tailfeathers turn the airplane, divided between vertical and horizontal parts of those depending on angle of bank.

 

[Jim Logajan]

No rudder, not much dihedral that I can see - this dang thing couldn't possibly turn:

 

[Jim Logajan]

Same way every other airplane turns, I imagine.

In mechanics I believe it is called a "couple".

 

[Jim Logajan]

Say what, what problem? the constant turn is caused by the force perpendicular to the the speed vector. I fail to see any 'problem'.

Two forces, no matter what their directions or magnitudes, acting on a single point can never cause a body attached to that point to rotate. You actually need two forces of equal magnitude operating in exactly opposite directions but acting on two different points to cause the body attached to those points to rotate. (If they are not equal and opposite they will also yield a translation.) The product of force magnitudes and distance is known as a couple in mechanics.

The sum of all forces on a body can be reduced to a force yielding pure translation and another yielding pure rotation. Any number of ways of generating rotational force in aircraft.

 

[Jim Logajan]

There is obvious dihedral, and quite a lot of it. Look at the bottom surface of the wing. Plus, that bird has thrust asymmetry to play with.

There are some videos on Youtube that show the B-2 in flight and it is interesting to watch the control surfaces as it makes banking turns. Looks more like an application of asymmetrical drag to handle yaw rather than engine thrust. The engines appear to be too close inboard to give decent yaw control, though no doubt that can be done.